Gases, in particular air are entrained in the pulp stock during the manufacture of paper. Entrained air or gases adversely affect the drainage of water from the pulp in the wet end of the paper machine. It is therefore, important to be able to accurately measure the amount of free gas in pulp stock at various stages of paper manufacture so as to be able to determine where in the system free gas enters the stock. Corrective measures can then be taken at appropriate places to minimize the amount of free gas entering the stock. The determination of the free gas content in a pulp also facilitates the measurement of the consistency by using a density measurement which is corrected for the free gas content in pulp.
The amount of air or gases in the stock can be measured by: measuring the density of the stock, removing all the gas from the stock, and measuring the volume removed; or by measuring the volume expansion of a stock sample by the reduction of pressure. Measuring the density of the stock to determine its free gas content is not very accurate. Removing all the free gas from a stock sample is time consuming and requires expensive equipment. The last method is the method commonly employed today. A known volume of a stock sample is subjected to a reduced pressure and the volume expansion of the sample is measured. One method of obtaining the reduced pressure is by using an electrically operated vacuum pump. Another method, developed at the Norwegian Pulp and Paper Institute, employs a pipette to hold a sample of pulp. A syringe burette is located below the pipette. Means are provided to connect the burette with the stock filled pipette, and a weighted piston in the burette is released for a short time to fall and create a pressure drop in the pipette. The distance the piston falls gives a measure of the air in the sample.
The known devices used to measure the amount of free gas in pulp stock, by measuring the volume expansion of a stock sample when the pressure is reduced, have disadvantages: The device employing a vacuum pump is cumbersome and requires careful control. The Norwegian Pulp and Paper Institute device has disadvantages affecting its accuracy. The loaded or weighted piston, used to create a pressure reduction in the stock has relatively high friction losses, and the volume measurement is not accurate, as the piston will tend to keep moving downward, as dissolved gases come out of solution. In addition, leakage occurs between the piston and adjacent walls leading to further inaccuracies.